This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Polo-like kinase 1 (Plk1) is a central mitotic kinase implicated in the regulation of the mitotic checkpoint, a mechanism controlling chromosomes segregation and preventing loss or gain of whole chromosomes, which is a known cause of birth defects and a contributing factor in the genesis of cancer. The multiplicity of Plk1 functions for has made it difficult to deconvolve its function in cell division. A potential approach to circumventing this problem is to identify kinetochore-specific PLK-1 substrates. We identified a series of N-terminal ?MELT? repeats in a protein called KNL-1 (Kinetochore Null protein-1) that fit the consensus site for PLK-1 phosphorylation. KNL-1 is a protein located on the chromosomes necessary for the assembly of the kinetochore and chromosomes segregation. We are testing the hypothesis that KNL-1 is a PLK-1 substrate and investigate the role of this phosphoregulation in chromosome segregation and checkpoint signaling. As predicted by the location of the MELT repeats, the N-terminal fragment of KNL-1 (KNL-11-505) is an excellent PLK-1 substrate in vitro (radioactive kinase assays). To assess the function of PLK-1 phosphorylation of KNL-1, in collaboration with the Scripps Research Institute Center for Physiological Proteomics, we mapped PLK-1 target sites on KNL-1 by using a combination of in vitro kinase assays and analysis by mass spectrometry. 6 phosphosites were each identified multiple times in the 3 samples we submitted to analysis, indicating that they are major sites on KNL-1 for PLK-1 phosphorylation. Now we are investigating the role of these phosphorylations in the control of mitosis progression and chromosomes segregation in C.elegans embryos.